Fork lift trucks are extensively employed to raise and lower objects of widely varying weight. For efficiency it is desirable that the lifting and lowering functions be accomplished at the highest speed consistent with safety. When dealing with relatively light loads no problems are generally encountered when either lifting or lowering the loads at the maximum speed available with the hydraulic system employed by the truck. However, as the weight of the load increases serious problems can be encountered if the lowering speed is not controlled within limits commensurate with the weight of the load.
Even though some operators can develop a rather delicate "touch" in the manipulation of the system control valve so as to provide a modicum of speed control by applying that tactile sensitivity to throttle the system valve during the lowering operation, it is quite dangerous to rely solely on the operator's skillful manipulation of the system control valve. Under heavy loads the lowering speeds can quickly approach dangerous levels. A moment's inattention can allow the load to reach dangerous momentum, and if the operator attempts to reduce the lowering speed too abruptly, the inertial forces of the lowering load can seriously damage the hoist mechanism, cause a displacement of the load and even tilt the truck itself, thereby endangering the life of the operator and anyone else in the immediate area.
Attempts have been made to provide flow control valves that automatically adjust the lowering speeds in relation to the size of the load being lowered. Initially, however, such attempts were relatively unsuccessful because the flow control valves were fully open at the inception of the lowering operation. During the time required for such valves to effect a modulation in the flow of the hydraulic fluid therethrough it was quite possible for the load to have already dropped an appreciable distance, thereby having gained a dangerous momentum before the flow control valve could become effective.
The first breakthrough in providing a relatively successful and safe flow control valve was with the particular valve disclosed in U.S. Pat. No. 3,414,007. That valve provided the first successful means by which to sense the load on the forks and adjust the flow rate through the valve in response to the load sensed, and it served as the industry standard for two decades with only modest changes being incorporated from time to time. The major deficiency of a pressure sensing valve made in conformity with that patent resides primarily in the fact that it responds with alacrity to an apparent, or transient, load as well as to an actual load. Hence, if the valve is subjected to an apparent decrease in loading at a time when the load is already being lowered, as would be the situation were the truck to engage a sudden drop in the surface over which it is traversing, the valve would immediately respond to the apparent load decrease and open, thus allowing the actual load to lower at in increased, and perhaps dangerous, rate of speed before the valve could respond to the true situation. Of less concern, but also a drawback was the rather loud hissing sound created by the unusual orifice pattern within that particular prior art valve.
Attempts were made to alleviate the undesirable sound, and the modifications which accomplished that end are disclosed in U.S. Pat. Nos. 3,421,545 and 3,433,253. In both arrangements a flap type valve member is employed that allows high flow rate when lifting a load, but which swings back into position where it blocks the orifice through the valve to the degree necessary to restrict flow, as required, to lower a heavy weight at the desired reduced rate of speed. Such valves adjust quite well to increasing or decreasing loads, so long as the sensing is done when there is no flow through the valve. However, as soon as flow is initiated, and a pressure differential occurs across the flap valve, it tends to bind against the orifice it is controlling and thereby inhibits the facile adjustment of the orifice should the load induced pressure change.
As a further disadvantage, all known prior art flow control valves have unusual exterior configurations which require that they be mounted, to a large degree, exteriorly with respect to the base of the cylinder with which they are operating, thus subjecting them to inadvertent physical abuse.